Ferritic stainless steel and use thereof in the manufacture for high temperature resistant products

ABSTRACT

Ferritic stainless steel, comprising the following Chemical elements expressed in percentage by weight: —Cr 14.0-20.0 —Al 0.50-1.50 —Zr 0.10-0.50 —Si 0.30-1.50 —Ti 0.10-0.35 —Nb 0.25-0.55 —C&lt;0.035 —N&lt;0.035 provided that the content of Ti, Nb, C and N satisfy the following relation: %Ti+%Nb/1.94&gt;9 (%C+%N) and substantially iron q. s. to 100. This steel may also comprise Yttrium and/or rare earth elements in a percentage by weight comprised in the range 0.10-0.30. The invention also encompasses products manufacturable with the steel of the invention, in particular vehicle exhaust manifold systems. Figure (I) shows a cyclic oxidation test carried out at 1000° C. on samples having the composition of an embodiment of the steel according to the invention.

[0001] The present invention refers to the field of the ferritic stainless steels usable in the manufacture of high temperature resistant products.

[0002] Object of the present invention is a ferritic stainless steel that, by virtue of its composition and of suitable thermo-mechanical treatments, is suitable for the manufacture of products exhibiting high performances at high temperatures, in particular for the manufacture of vehicle exhaust manifold systems.

[0003] In the manufacture of vehicle exhaust system components, ferritic stainless steels, by virtue of their smaller thermal expansion coefficient, are preferred to the austenitic ones. In the development of the related technological field, ferritic stainless steels are adopted for the manufacture of vehicle exhaust system components working at temperatures reaching 750° C. For the manufacture of a manifold, subjected to temperatures higher than 800° C., cast iron had longtime been preferred to ferritic stainless steels. However, the adoption of the former material proved unsatisfactory due to the occurrence of drawbacks related to the weight increases, the entailed fuel consumption, and the design difficulty due to the high thickness of the component thus manufactured.

[0004] Attempts at replacing cast iron with ferritic stainless steels for this specific use yielded fair results as to the lightness of weight of the product. However, the limitations due to the inadequacy of performance of the materials advanced, in the presence of exposure to high-temperature exhaust gases and to repeated thermal shocks under oxidizing conditions, especially with regard to spalling, still await overcoming.

[0005] Hence, in the specific field there is a demand for a ferritic stainless steel suitable for the manufacture of manifolds exhibiting performances free from the drawbacks of the prior art materials.

[0006] In fact, object of the present invention is a ferritic stainless steel, comprising the following chemical elements expressed in percentage by weight: Cr 14.0-20.0 Al 0.50-1.50 Zr 0.10-0.50 Si 0.30-1.50 Ti 0.10-0.35 Nb 0.25-0.55 C <0.035 N <0.035

[0007] provided that the content of Ti, Nb, C and N satisfy the following relation:

%Ti+%Nb/1.94>9 (%C+%N)

[0008] and substantially iron q.s. to 100.

[0009] The ferritic stainless steel according to the present invention may also comprise Yttrium and/or rare earth elements in a percentage by weight comprised in the range 0.10-0.30.

[0010] The present invention further encompasses strips or sheets made of steel according to the invention, weldable, formable and highly resistant to oxidation and creep.

[0011] A further object of the present invention is a process for the preparing of the strips or sheets, wherein the steel according to the invention is subjected to the following steps:

[0012] hot and/or cold rolling;

[0013] annealing, after hot and/or cold rolling at a temperature ranging from 900° to 1200° C. and for time lower than 3600s;

[0014] optional pickling after hot and/or cold rolling.

[0015] The present invention encompasses the use of the ferritic stainless steel according to the invention for the manufacture of products in form of flat bloom, strip, ingot, casting, forging or semimanufactured item, as well as the flat blooms, the strips, the ingots, the castings, the forgings and the semimanufactured items made therewith.

[0016] The present invention further refers to the use of the steel strip or sheet according to the invention for the production of pipes or of pipe-derivable products.

[0017] The pipes thus manufactured can be weldless or welded, even longitudinally welded. Also the pipes and the pipe-derivable products manufactured with said steel strips or sheets are an object of the present invention.

[0018] Lastly, the invention also refers to high temperature exhaust system components, in particular to vehicle exhaust manifolds.

[0019] At high temperatures the ferritic stainless steel of the invention exhibits the desired features, in particular a resistance to hot cyclic oxidation, by virtue of an apt synergy of chemical elements like Chromium, Aluminum, Zirconium, Silicon, Titanium, Niobium, Carbon, Nitrogen and optionally Yttrium and/or rare earth elements. These features, desired at high temperatures, are enhanced by the hot and/or cold rolling and by the subsequent annealing treatment of sheets and/or strips thus obtainable, during which a Zircon carbonitride precipitation occurs, optionally followed by a pickling treatment.

[0020] The Inventors deem the functions of the main chemical elements forming the alloy of the present invention to be as follows.

[0021] Chromium, in the indicated percentages by weight, remarkably increases resistance to oxidation without promoting the formation of brittle phases.

[0022] Silicon and Aluminum stabilize the ferritic matrix and are accountable for the increase of resistance to hot oxidation. An employ of these elements above the upper limits set forth is detrimental to steel quality in terms of formation of intermetallic phases and of increased manufacturing problems.

[0023] Zircon forms stable carbonitrides, whose presence influences the microstructural development during annealing of the cold-rolled items.

[0024] Titanium and Niobium form stable carbides and nitrides, and prevent the precipitation of Chromium nitrides and carbides onto the edge of the grains and the entailed dechroming (sensitizing) of the matrix in the vicinities thereof, further promoting the presence of an entirely ferritic structure at all temperatures, as reducing the dissolved Carbon and Nitrogen quantities.

[0025] Yttrium and/or rare earth elements enhance the resistance to hot oxidation in the presence of thermal and mechanical shocks due to the increased adhesion of the oxide onto the metal substrate.

[0026] So far, the present invention was generally described. With the aid of the figures and of the examples, hereinafter a more detailed description of specific embodiments of the invention will be given, aimed at making apparent the objects, the features, the advantages and the application modes thereof.

[0027]FIG. 1 shows the weight variation per surface unit effected on samples of austenitic and of ferritic steels in order to identify a steel known to the state of the art apt to provide the best features in terms of resistance to hot cyclic oxidation, carrying out for this purpose a cyclic oxidation test according to the number of cycles at temperatures equal to 1000° C.

[0028]FIG. 2 shows the weight variation per surface unit in an AISI 441 steel sample of a composition known to the art, identified as apt to provide the best features to hot cyclic oxidation, carrying out for this purpose a cyclic oxidation test according to the number of cycles at temperatures equal to 1000° C.

[0029]FIG. 3 shows the weight variation per surface unit in a steel sample according to the invention, carrying out for this purpose a cyclic oxidation test according to the number of cycles at temperatures equal to 1000° C.

EXAMPLE 1

[0030] In order to evaluate the resistance to thermal cycling under critical operating conditions for steel members positioned at the hot portion of vehicle exhaust systems, these members were subjected to cyclic oxidation resistance tests.

[0031] The latter were carried out on known steels at various temperatures in order to determine the known steel type better meeting the thermal cycling resistance requirements.

[0032] A standard test procedure was adopted, with 25 min heating/in-oven permanence and 5 min spontaneous air cooling cycles.

[0033] Ferritic stainless steels 441, 436, 429, and austenitic steels 321, 309, 310, 4828 operating at a 1000° C. temperature were tested.

[0034]FIG. 1 shows the weight variation per surface unit of the various steel types according to the number of cycles.

[0035] Apparently, austenitic steels exhibit a worse behavior, with a swift weight decrease due to spalling. In particular, steel 321 has a high instability and decays over a very short time.

[0036] On the contrary, results concerning ferritic steels are more satisfactory, as the weight remains almost unvaried during all the oxidation cycles.

[0037] In particular, steel 441 undergoes a slight weight increase, with respect to 436 and 429 steels that instead exhibit a slight weight decrease after the test. Hence, the steel 441, apt to provide the best performances among known steels, was used as comparison to the steel advanced according to the present invention.

[0038] This conventional steel AISI 441 was prepared according to the following composition expressed as percentage by weight: C 0.016; Cr 18.18; Si 0.60; Al 0.09; Nb 0.42; Ti 0.18; Mn 0.18; Cu 0.08; N 0.012, and iron q.s. to 100.

[0039] According to this composition, there was obtained a plane product in form of flat bloom, that was heated to 1150°, and yielded, by subsequent hot rolling, strips of a 5 mm thickness that were subsequently annealed 1 min at 1050° C.

[0040] Post pickling, the hot strips were forwarded to cold rolling down to a 1.5 mm thickness and then annealed 40 sec at a 1080° C. temperature.

[0041] The resistance to cyclic oxidation of the strip thus obtained was tested at 1000° C. and at 1050° C., in a 1000-cycle test, each cycle consisting of 25 min heating /in-oven permanence and 5 min air cooling. Test results are shown in FIG. 2.

[0042] Onto the same hot-rolled and subsequently cold-rolled strip SAG tests were carried out for 100 hours at a 1000° C. temperature in order to evaluate the creep resistance thereof. The SAG test result is yielded by measuring the permanent deflection after in-oven exposure of the samples suspended by the ends thereof.

[0043] The end-test measuring highlighted a permanent deflection of over 40 mm, well beyond the 18 mm threshold generally adopted by the current technology.

EXAMPLE 2

[0044] A steel according to the present invention is prepared, having the following composition expressed as percentage by weight: Cr 17.68; Al 0.94; Zr 0.15; Si 1.16; Ti 0.21; Nb 0.40; C 0.022; N 0.013, and substantially iron q.s. to 100.

[0045] The steel is cast by a casting process into flat blooms having thin thicknesses typically ranging from 50 to 90 mm, and in the example being equal to 60 mm. This flat bloom obtained according to the abovedisclosed composition was heated to 1150° C., and by a subsequent hot rolling thereof strips having a thickness equal to 5 mm were produced.

[0046] The strips thus obtained were annealed 1 min at 1050° C.

[0047] Post pickling, the hot strips were forwarded to cold rolling down to a 1.5 mm thickness and then annealed 40 sec at a 1080° C. temperature.

[0048] The resistance to cyclic oxidation of the strip thus obtained was tested at 1000° C. and at 1050° C., in a 1000-cycle test, each cycle consisting of 25 min heating/in-oven permanence and 5 min air cooling. The test results are shown in FIG. 2.

[0049] Onto the same hot- and cold-rolled strip, SAG tests were carried out for 100 hours at a 1000° C. temperature in order to evaluate the creep resistance thereof.

[0050] The SAG test result in terms of permanent deflection measuring after in-oven exposure of the strip samples suspended by the ends thereof yielded the value of 11 mm.

[0051] Hence, apparently the steel according to the invention remains below the 18 mm deflection threshold, generally adopted by the current technology.

EXAMPLE 3

[0052] A steel according to the present invention is prepared, having the following composition expressed as percentage by weight:

[0053] Cr 18; Al 0.94; Zr 0.15; Si 0.95; Ti 0.18; Nb 0.44; C 0.015; N 0.013, further comprising Yttrium and 0.20% b/w rare earth elements, and substantially iron q.s. to 100.

[0054] The steel was cast by a traditional casting process into flat blooms that were first heated to 1150° C. Then, by a subsequent hot-rolling, 5 mm thick strips were made.

[0055] The strips thus obtained were annealed 1 min at 1050° C.

[0056] Post pickling, the hot strips were forwarded to cold rolling, down to a 1.5 mm thickness, and then annealed 40 sec at a 1080° C. temperature.

[0057] The resistance to cyclic oxidation of the strip thus obtained was tested at 1000° C. and at 1050° C., in a 1000-cycle test, each cycle consisting of 25 min heating/in-oven permanence and 5 min air cooling. The test results are substantially equivalent to those shown in FIG. 3.

[0058] Onto the same hot-rolled and subsequently cold-rolled strip, SAG tests were carried out for 100 hours at a 1000° C. temperature in order to evaluate the resistance to creep thereof. The post-test measuring highlighted a permanent deflection of about 13 mm, below the 18 mm deflection threshold generally adopted by the current technology. 

1. A ferritic stainless steel, which comprises the following chemical elements expressed in percentage by weight: Cr 14.0-20.0 Al 0.50-1.50 Zr 0.10-0.50 Si 0.30-1.50 Ti 0.10-0.35 Nb 0.25-0.55 C <0.035 N <0.035

provided that the contents of Ti, Nb, C and N satisfy the following relation: %Ti+%Nb/1.94>9 (%C+%N) and substantially iron q.s. to
 100. 2. The ferritic stainless steel according to claim 1, and further including Yttrium and/or rare earth elements in a percentage by weight comprised in the range 0.10-0.30.
 3. A steel strip or sheet, made of ferritic stainless steel according to claim 1, which is weldable, formable, and highly resistant to oxidation and creep.
 4. A process for preparing the steel strip or sheet according to claim 3, which comprises the following steps: hot and/or cold rolling; annealing, after hot and/or cold rolling, at a temperature ranging from 900° to 1200° C. and for times lower than 3600s; optional pickling after hot and/or cold rolling.
 5. (Cancelled)
 6. Flat blooms, strips, ingots, castings, forgings and semimanufactured items, made of steel according to claim
 1. 7. (Cancelled)
 8. (Cancelled)
 9. Pipes and pipe-derivable products, manufactured with the strip or sheet according to claim
 3. 10. The pipe-derivable product according to claim 9, employed as a component of a high temperature exhaust system.
 11. The pipe derivable product according to claim 10, employed as a manifold of a vehicle exhaust system.
 12. Weldless pipes, welded pipes or longitudinally welded pipes manufactured with the steel strip or sheet according to claim
 3. 13. A steel strip or sheet, made of steel according to claim 2, which is weldable, formable and highly resistant to oxidation and creep.
 14. A process for preparing the steel strip or sheet according to claim 13, which comprises the following steps: hot and/or cold rolling annealing, after hot and/or cold rolling, at a temperature ranging from 900° to 1200° C. and for times lower than 3600s; optional pickling after hot and/or cold rolling.
 15. Flat blooms, strips, ingots, castings, forgings and semimanufactured items made of steel according to claim
 2. 